(1) Field of the Invention
This invention relates to a semiconductor light-emitting element, particularly usable for a white light-emitting diode.
(2) Related Art Statement
Recently, various light-emitting diodes (LEDs) are widely available. LEDs are expected for illuminating use as well as displaying use because of their low electric power consumption, long life time, CO2 gas reduction originated from the reduction of the high energy consumption such as the low electric power consumption, and thus, much demand for the LEDs are expected.
As of now, the LEDs are made of various semi-conducting material such as GaAs-based semi-conducting material, AlGaAs-based semi-conducting material, GaP-based semi-conducting material, GaAsP-based semi-conducting material and InGaAlP-based semi-conducting material, and thus, can emit various color lights from red to yellow-green. Therefore, the LEDs are employed particularly for various displaying use. Recently, blue and green LEDs have been realized by using GaN-based semi-conducting material. As a result, selecting a given LED, a given color light from red to blue, that is, within visible light range, can be obtained from the LED, and full-color displaying is also realized. Moreover, white light-emitting diodes (white LEDs) are being realized by using RGB LED chips or using two color lights-emitting diodes composed of blue LEDs with yellow fluorescent substance thereon. As a result, LED illumination is being realized at present.
However, the white LED using the RGB LED chips requires higher cost because the plural LED chips are employed, so that in view of the cost, it is difficult to employ the white LED for illumination use. On the other hand, full color can not be recognized by the white LED using the two color lights-emitting diode because it employs only two primary colors, not three primary colors. Moreover, in the white LED, the brightness of only about 25 lm/w can be realized, which is very small as compared with the brightness of 90 lm/W of a fluorescent tube.
Therefore, a white LED employing three primary colors is strongly desired all over the world because of the low energy consumption taking environmental problem into consideration. In reality, such a white LED is intensely developed by Japanese national professions and foreign major electric-manufacturing enterprises.
Such an attempt is made as to fabricate a white LED using three or over primary colors as illuminating a three primary colors-fluorescent substance by an ultraviolet LED. This attempt is fundamentally based on the same principle as a fluorescent tube, and employs the ultraviolet LED as the ultraviolet beam from the mercury discharge in the fluorescent tube. In this case, the cost of the white LED is increased because the three primary colors-fluorescent substance is additionally employed for the ultraviolet LED. Using a GaN-based semi-conducting material, a blue LED can be realized, and then, using the GaN-based semi-conducting material, the ultraviolet LED can be realized. However, the luminous efficiency of the resulting ultraviolet LED is largely reduced, as compared with the blue LED.
The luminescence reduction is considered as follows. If the GaN-based semiconductor film is epitaxially grown on a substrate made of e.g., a sapphire single crystal, much misfit dislocations are created at the boundary between the film and the substrate due to the difference in lattice constant between the film and the substrate. The misfit dislocations are propagated in the film and a light-emitting layer provided on the film, and thus, many dislocations are created in the resulting LED.
In a blue LED or a green LED made of GaN-based semi-conducting materials, the light-emitting layer is made of an InGaN semi-conducting material. In this case, the In elements are partially located, and thus, some carriers are located and confined. Therefore, the carriers are recombined before they are moved and seized at the dislocations, so that the LED can exhibit its sufficient luminous efficiency.
That is, even though much dislocation are created in the light-emitting layer, the carriers are recombined and thus, a given luminescence is generated before they are moved and seized at the dislocation as non-luminescence centers, so that the blue LED or the green LED using the GaN-based semi-conducting materials can exhibit their high luminous efficiency.
For fabricating an ultraviolet LED, the In ratio of the light-emitting layer must be reduced. Therefore, the In elements are not almost located, and thus, the diffusion length of carrier is elongated. As a result, the carriers are easily moved at and recombined with the dislocations in the light-emitting layer. In this way, the luminous efficiency of the ultraviolet LED is reduced due to the large amount of dislocation in the light-emitting layer, as compared with the blue LED. In this point of view, various dislocation-reducing method are researched and developed.
For example, such an ELO technique is proposed as fabricating a strip mask made of SiO2 during an epitaxial process and preventing the propagation of the misfit dislocations created at the boundary between the epitaxial film and a substrate. According to the ELO technique, a light-emitting layer having fewer dislocations can be formed above the substrate via strip mask. However, the ELO technique is a complicated means, so that the manufacturing cost is increased. Then, in the ELO technique, a thicker layer made of e.g., a GaN-based semi-conducting material is formed on the substrate, which results in being curved. Practically, in a device manufacturing process, when epitaxial films are formed on their respective substrates by the ELO technique, the better half of the substrates is broken. Therefore, it is difficult to employ the ELO technique in a practical device manufacturing process, particularly for LEDs.
In addition, an attempt is made to epitaxially grow a bulky GaN single crystal for reducing the dislocation density of the resulting device, for example by using a high pressure solution growth method, a vapor phase epitaxial growth method or a flux method. As of now, however, such a bulky single crystal enough to be applied for the device manufacturing process is not grown and prospected.
For fabricating a bulky GaN single crystal of low dislocation density, an attempt is made to grow a thicker GaN single crystal on a substrate made of an oxide to match in lattice the GaN single crystal by a HVPE method, and thereafter, remove the substrate, to obtain only the GaN single crystal to be used as a substrate. However, the GaN single crystal enough to be industrially applied for LEDs has not been fabricated yet.
As a result, the high luminous efficiency in such a white LED as employing three or over primary colors through the illumination of a fluorescent substance by an ultraviolet LED is not technically prospected.
It is an object of the present invention to provide a new semiconductor light-emitting element preferably usable for a LED to emit an any color light regardless of the dislocation density, particularly a white LED.
For achieving the above object, this invention relates to a semi-conductor light-emitting element including a substrate, an underlayer, formed on the substrate, made of a first semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a first conductive layer, formed on the underlayer, made of a second semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a first cladding layer, formed on the first conductive layer, made of a third semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a light-emitting layer composed of a base layer, formed on the first cladding layer, made of a fourth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In and plural isolated island-shaped single crystal portions, embedded in the base layer, made of a fifth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In and having an in-plane lattice constant larger than that of the third semi-conducting nitride material, a second cladding layer, formed on the light-emitting layer; made of a sixth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, and a second conductive layer, formed on the second cladding layer, made of a seventh semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In.
Then, the bandgap of the third semi-conducting nitride material constituting the first cladding layer, the bandgap of the fourth semi-conducting nitride material constituting the base layer and the bandgap of the fifth semi-conducting nitride material become larger by turns. That is, the relation of the handgap of the third semi-conducting nitride material greater than the handgap of the fourth semi-conducting nitride material greater than the handgap of the fifth semi-conducting nitride material is satisfied. Moreover, at least one element selected from the group consisting of rare earth metal elements and transition metal elements is incorporated, as an additive element, into at least one of the base layer and the plural island-shaped single crystal portions, to generate and emit an any color light from the whole of the light-emitting layer.
Recently, such a LED as to be illuminated through a light-emitting layer having mismatched in lattice and isolated island-shaped single crystal portions have been intensely researched and developed. The inventors has paid attention to the potential performance of the semiconductor light-emitting element having the light-emitting layer with island-shaped single crystal portions, and made an attempt to realize the element by controlling the manufacturing condition.
Regardless of the above attempt, however, the semiconductor light-emitting element can not exhibit the practical luminescence intensity and a desired color light, particularly, a white color. Therefore, the inventors had conceived that at least one element selected from the group consisting of rare earth metal elements and transition metal elements is incorporated as an additive element into the at least one of the island-shaped single crystal portions and the base layer to cover and support the island-shaped portions which constitute the light-emitting layer.
When the rare earth metal elements and the transition metal elements are excited by an external energy, a given inherent wavelength light (fluorescent light) is emitted from each of the elements. Therefore, if a rare earth metal element or a transition metal element is incorporated into the island-shaped single crystals and/or the base layer constituting the light-emitting layer, a given inherent wavelength light (fluorescent light) is emitted from the element through the excitation of the light emitted from the island-shaped single crystal portions.
Accordingly, if a rare earth metal element or a transition metal element is appropriately selected, an any color light, particularly, a white light can be easily obtained by taking advantage of the fluorescent light of the rare earth element or the transition element. In view of easy excitation and easy creation of any color light through the inherent fluorescent light, a rare earth metal element is preferably used.
Moreover, if different rare earth metal elements, that is, Tm element to generate a blue color wavelength-region light, Er element to generate a green color wavelength-region light and Eu or Pr element to generate a red color wavelength-region are incorporated into the light-emitting layer, a white light can be obtained only through the superimposition of the fluorescent lights from the rare earth metal elements.
The above invention is realized on the vast and long term research and development as mentioned above.
This invention also relates to A semiconductor light-emitting element including a substrate, an underlayer, formed on the substrate, made of a first semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a first conductive layer, formed on the underlayer, made of a second semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a first cladding layer, formed on the first conductive layer, made of a third semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a light-emitting layer composed of a base layer, formed on the first cladding layer, made of a fourth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In and plural isolated island-shaped single crystal portions, embedded in the base layer, made of a fifth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In and having an in-plane lattice constant larger than that of the third semi-conducting nitride material, a second cladding layer, formed on the light-emitting layer, made of a sixth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, and a second conductive layer, formed on the second cladding layer, made of a seventh semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In. Then, the bandgap of the third semi-conducting nitride material constituting the first cladding layer, the bandgap of the fourth semi-conducting nitride material constituting the base layer and the bandgap of the fifth semi-conducting nitride material become larger by turns. That is, the relation of the handgap of the third semi-conducting nitride material greater than the handgap of the fourth semi-conducting nitride material greater than the handgap of the fifth semi-conducting nitride material is satisfied. Moreover, at least one element selected from the group consisting of rare earth metal elements and transition metal elements is adsorbed, as an adsorbing element, onto at least one of the boundary between the base layer and the island-shaped single crystal portions and the boundary between the base layer and the first cladding layer.
For developing the luminescence-recombination probability of electron and electron hole in the island-shaped single crystal portions, it is required that the volume of each of the island-shaped single crystal portions is decreased as small as possible, and the electrons and the electron holes, flown from the outside, are confined in the island-shaped single crystal portions strongly. That is, as the volumes of the island-shaped single crystal portions are decreased, the luminescence probability is increased.
However, as the volume of the island-shaped single crystal portion is decreased, the surface area thereof is increased, so that the number of the dangling bond of the island-shaped single crystal portion is increased. The carriers of electrons and electron holes are recombined with the dangling bonds in non-luminescence, and thus, the absolute number of carrier to be recombined in luminescence is decreased.
Therefore, if, according the second semiconductor light-emitting element, a rare earth metal elements and/or a transition metal elements is adsorbed at the boundary between the base layer and the island-shaped single crystal portions and/or the boundary between the base layer and the first cladding layer, and thus, the dangling bonds at the surfaces of the island-shaped single crystal portions are reduced, the recombination in non-luminescence can be reduced and a desired color light can be obtained at a sufficient efficiency.
The mode of the second semiconductor light-emitting element may be employed in isolation or combination with the mode of the above-mentioned first semiconductor light-emitting element. In the combination case, the fluorescent lights from the rare earth metal element and the transition element can be enhanced, and thus, a desired color light can be easily generated. Also, from the similar reason to the one of the first semiconductor light-emitting element, a rare earth metal element is preferably employed.
In the combination of the modes of the first and the second light-emitting elements, it is desired that the kind of the additive element into the base layer or the like constituting the light-emitting layer is the same as the kind of the adsorbing element onto the boundary between the island-shaped single crystal portions and the base layer, or the like. In this case, a desired color light can be generated at a higher intensity.
Particularly, if, as adsorbing elements, the same Tm element to generate a blue color wavelength-region light, the same Er element to generate a green color wavelength-region light and the same Eu or Pr element to generate a red color wavelength-region light are employed as the additive elements, a white light can be easily generated at a higher intensity.
In the semiconductor light-emitting element of the present invention, the in-plane lattice constant of the fifth semi-conducting nitride material constituting the island-shaped single crystal portion is set to be larger than the in-plane lattice constant of the third semi-conducting nitride material constituting the first cladding layer. In this case, compression stress is affected on the fifth semi-conducting nitride material, which results in being shaped in dot. That is, the island-shaped single crystal portions are formed on the compressive stress.
The bandgap arrangement in the third semi-conducting nitride material constituting the first cladding layer, the fourth semi-conducting nitride material constituting the base layer and the fifth semi-conducting nitride material constituting the island-shaped single crystal portions is required to confine the island-shaped single crystal portions energetically and emit a given wavelength light from each of the single crystal portions.